In the purest form of the reaction, fuel cells produce electricity from hydrogen and oxygen with water being produced as a by-product in the form of steam. However, hydrocarbon fuels such as natural gas or higher (C2+) hydrocarbons are commonly used as the source of hydrogen and air is used as the source of oxygen.
Prior to delivery to the fuel cell, it is now conventional to process hydrocarbon fuel to a lesser or greater extent using a fuel reformer. Thus, in proton exchange membrane (PEM) type fuel cells, it is intended that the hydrocarbon fuel undergoes substantially complete ireformation by reaction with water (steam) in order to produce a hydrogen-rich stream for delivery to the fuel cell. In contrast, in solid oxide fuel cell (SOFC) systems it is possible to use catalysts within the fuel cell itself (on the anode side of the fuel cell) to effect reforming of hydrocarbons (usually methane). So-called internal reforming in this way has advantages for operating efficiency in terms of balancing the exothermic electricity-generating reactions that occur within the fuel cell with the endothermic reforming reaction. However, in this case the fuel composition to the fuel cell and the extent of internal reforming within the cell should be controlled to avoid excessive cooling of the fuel cell. In practice, where a fuel cell is designed to carry out internal reforming, the fuel to be delivered to it is pre-processed in a fuel reformer in order to manipulate the hydrocarbon content of the fuel as required based on the operating characteristics of the cell. Here the reformer is typically referred to as a steam pre-reformer.
Hydrocarbon reforming takes place in the presence of steam, and the steam to carbon ratio in the gas stream to the reformer is one of the most critical variables in the reforming reaction. Furthermore, the presence of steam in the fuel stream to the fuel cell can prevent carbon deposition on the catalyst used to effect internal reforming. Accurate control of the steam to carbon ratio is therefore an important consideration.
It is also important for effective and efficient operation of a fuel reformer that the steam and fuel to be processed are delivered at a suitable rate/pressure. A non-uniform hydrocarbon flow rate will lead to changing fuel utilisation and this may be detrimental to the fuel cell stack of the system. If a steam venturi (ejector) is used to entrain the hydrocarbon fuel, a change in the steam flow rate to the venturi will immediately change the fuel uptake/flow rate. If natural gas is delivered by a blower for example, a change in steam flow rate will change the back-pressure of the system and this will alter the fuel flow rate to the system.
Invariably, in conventional systems the steam is delivered to the reformer under pressure from a steam generator. The steam flow rate that is required is very low by normal industrial standards (typically it is about 1 kg/hr at most for a 2 kW fuel cell system) and to account for this the steam generator typically consists of a number of small bore tubes that are supplied with water and heated. This approach for generating steam is generally referred to as flow boiling.
Whilst generally useful, production of steam in this manner can lead to pressure and flow fluctuations (or pulses) and, often, reverse flow in the tubing used for generating the steam. These effects have been reported in relation with the use of mini-channels (200 μm-3 mm diameter) and micro-channels (<200 μm diameter) for steam generation. In the case of mini-channels the oscillatory nature with which the steam is generated and the occurrence of reverse flow are believed to be due to a variety of mechanisms with the nucleation of bubbles being a significant factor. More particularly, three flow patterns are believed to be common to flow boiling in mini-channels: isolated bubble flow; confined bubble flow; and annular-slug flow (see S. G. Kandlikar, Fundamental issues related to flow boiling in mini-channels and micro-channels, Experimental Thermal and Fluid Science 26 (2002) pp 389-407). The mechanism(s) in operation for flow boiling in micro-channels is less well understood although surface tension effects are believed to be significant (see the article by S. G. Kandlikar as referenced).
Irrespective of the reasons why pressure fluctuations and reverse flow occur, when steam is generated in the manner described these effects can present problems in fuel cell systems. In addition to the abovementioned non-steady entrainment of fuel in a steam venturi used to entrain the fuel, pressure pulses associated with steam production may extend throughout the fuel cell system and adversely affect components as a result. For example, pressure pulses associated with steam generation can be experienced as far downstream in the fuel cell system as the burner used to burn (anode) exhaust from the fuel cell, and here the pulses can cause problems with consistency of burner operation.
Against this background, it would be desirable to provide a method of generating steam in a stable manner and at a more uniform flow rate than provided by existing steam generators. Such a method would not suffer the disadvantages described above.